The invention relates to a tire pressure sensor for motor vehicles.
It is known from DE-OS No. 28 32 447 to screw in a tire pressure sensor at the circumference of a wheel rim for monitoring the inflation pressure in the tire, which tire pressure sensor comprises a pressure switch which is to be actuated by the tire pressure. The pressure switch comprises, on one hand, an electrically conductive diaphragm which closes a reference pressure chamber in the direction of the tire and, on the other hand, a contact pin which is sealed in a glass bushing in the reference pressure chamber so as to be electrically insulated. The middle area of the diaphragm contacts the contact pin so as to close an electric circuit when there is sufficient inflation pressure in the tire. The reference pressure chamber is filled via a duct with air or nitrogen to a desired reference pressure and is then sealed so as to be tight against pressure. The reference pressure is selected in such a way that the diaphragm contacts the end of the contact pin when there is sufficient inflation pressure in the tire. However, if the inflation pressure in the tire falls below a minimum pressure predetermined by the reference pressure the diaphragm is lifted from the contact pin and opens a resonant circuit which comprises a coil and a capacitor and is arranged in the tire pressure sensor. It can be determined whether or not the pressure switch is closed by a signal receiver arranged in the vicinity of the wheel, every time the tire pressure sensor runs past at every revolution of the wheel via a high-frequency oscillation of the evaluating circuit which is fixedly arranged in the vehicle.
Such known constructions have the disadvantage that the reference pressure chamber is filled with the gas during the manufacturing of the tire pressure sensor, the sensor is adjusted to a predetermined reference pressure, and the reference pressure chamber is then welded so as to be gastight, so that it is no longer possible to subsequently adapt the reference pressure to different values which are predetermined by the type of vehicle, by the tire, the speed or the loading of the vehicle. Therefore, a quantity of pressure sensors with different tire pressures must be produced and kept in storage.
In another solution known from DE-PS No. 33 10 052, the reference pressure in the reference pressure chamber of the tire pressure sensor is adjustable in a variable manner in that the tire pressure is fixed simultaneously as new reference pressure for the reference pressure chamber via a valve arrangement during the tire filling process. However, this has the disadvantage that the valve arrangement does not ensure the sealing of the reference pressure chamber necessary for the life of the tire pressure sensor so that, in the event of a leaky valve, air escapes from the reference pressure chamber when a difference in pressure occurs between the reference pressure chamber and the tire, which defect is not noticed. An insufficient tire pressure during a gradual drop in pressure in the tire is then no longer indicated.